U.S. Pat. No. 9,127,431 discloses a safe cover of an inspection chamber containing an at least partial coating made of non-metallic material conducting radio signals. This coating is installed on the framework of the manhole, such that it covers the inspection chamber opening. The cover is provided with a sabotage sensor detecting changes of its location, and with a transmitter. The transmitter is configured to generate a radio frequency signal if the sensor detects the pre-set motion of the manhole cover coating. A mobile connection is provided between an antenna and the transmitter, allowing radio frequency energy to be generated by the manhole cover coating and sent to a receiver located outside the manhole.